A variety of liquid crystal display elements (LCD) including a twisted nematic (TN) mode, a super twisted nematic (STN) mode, a thin-film transistor (TFT) mode or the like have been put to practical use, because of having lower power consumption, permitting smaller and lighter as compared with a CRT (cathode-ray tube mode display). Among them, an active-matrix LCD (AM-LCD) such as TFT or the like are getting colored and sophisticated, to which attention has been drawn as a prospective flat display.
The characteristics required for the liquid crystal compositions for AM-LCD are recited below.
1) A voltage holding ratio (V.H.R.) is high to maintain high contrast of LCD. PA0 2) A nematic liquid crystal phase range is broad depending on the environment in which the liquid crystal composition is used. PA0 3) A suitable optical anisotropy (.DELTA.n) is acceptable depending on the cell thickness; and PA0 4) A suitable threshold voltage is acceptable depending on the driver. PA0 3,4,5-trifluorophenyl-(2-fluoro-4-(trans-4-methylcyclohexyl))benzoate; PA0 3,4,5-trifluorophenyl-(2-fluoro-4-(trans-4-ethylcyclohexyl))benzoate, Cr 79.3-79.6 Iso; PA0 3,4,5-trifluorophenyl-(2-fluoro-4-(trans-4-butylcyclohexyl))benzoate; PA0 3,4,5-trifluorophenyl-(2-fluoro-4-(trans-4-pentylcyclohexyl))benzoate, Cr 73.8-74.7N 85.0-85.1 Iso; PA0 3,4,5-trifluorophenyl-(2-fluoro-4-(trans-4-hexylcyclohexyl))benzoate; PA0 3,4,5-trifluorophenyl-(2-fluoro-4-(trans-4-heptylcyclohexyl))benzoate; PA0 3,4,5-trifluorophenyl-(2-fluoro-4-(trans-4-octylcyclohexyl))benzoate; PA0 3,4,5-trifluorophenyl-(2-fluoro-4-(trans-4-nonylcyclohexyl))benzoate; PA0 3,4,5-trifluorophenyl-(2-fluoro-4-(trans-4-decylcyclohexyl))benzoate. PA0 2,6-difluoro-4-(trans-4-alkylcyclohexyl)benzonitrile, in place of 2-fluoro-4-(trans-4-propylcyclohexyl)benzonitrile. PA0 3,4,5-trifluorophenyl-(2,6-difluoro-4-(trans-4-methylcyclohexyl))benzoate; PA0 3,4,5-trifluorophenyl-(2,6-difluoro-4-(trans-4-ethylcyclohexyl))benzoate; PA0 3,4,5-trifluorophenyl-(2,6-difluoro-4-(trans-4-propylcyclohexyl))benzoate; PA0 3,4,5-trifluorophenyl-(2,6-difluoro-4-(trans-4-butylcyclohexyl))benzoate; PA0 3,4,5-trifluorophenyl-(2,6-difluoro-4-(trans-4-pentylcyclohexyl))benzoate Cr 76.8-77.1(N 63.1-63.8) Iso; PA0 3,4,5-trifluorophenyl-(2,6-difluoro-4-(trans-4-hexylcyclohexyl))benzoate; PA0 3,4,5-trifluorophenyl-(2,6-difluoro-4-(trans-4-heptylcyclohexyl))benzoate; PA0 3,4,5-trifluorophenyl-(2,6-difluoro-4-(trans-4-octylcyclohexyl))benzoate; PA0 3,4,5-trifluorophenyl-(2,6-difluoro-4-(trans-4-nonylcyclohexyl))benzoate; PA0 3,4,5-trifluorophenyl-(2,6-difluoro-4-(trans-4-decylcyclohexyl))benzoate. PA0 3,4,5-trifluorophenyl-(2-fluoro-4-(trans-4-(trans-4-methylcyclohexyl)cycloh exyl)benzoate; PA0 3,4,5-trifluorophenyl-(2-fluoro-4-(trans-4-(trans-4-propylcyclohexyl)cycloh exyl))benzoate, Cr 110.1-110.9N 248.9-249.7 Iso; PA0 3,4,5-trifluorophenyl-(2-fluoro-4-(trans-4-(trans-4-(trans-4-butylcyclohexy l)cyclohexyl))benzoate; PA0 3,4,5-trifluorophenyl-(2-fluoro-4-(trans-4-(trans-4-(trans-4-pentylcyclohex yl)cyclohexyl))benzoate; PA0 3,4,5-trifluorophenyl-(2-fluoro-4-(trans-4-(trans-4-hexylcyclohexyl)cyclohe xyl)benzoate; PA0 3,4,5-trifluorophenyl-(2-fluoro-4-(trans-4-(trans-4-heptylcyclohexyl)cycloh exyl)benzoate; PA0 3,4,5-trifluorophenyl-(2-fluoro-4-(trans-4-(trans-4-octylcyclohexyl)cyclohe xyl))benzoate; PA0 3,4,5-trifluorophenyl-(2-fluoro-4-(trans-4-(trans-4-nonylcyclohexyl)cyclohe xyl))benzoate; PA0 3,4,5-trifluorophenyl-(2-fluoro-4-(trans-4-(trans-4-decylcyclohexyl)cyclohe xyl)benzoate. PA0 3,4,5-trifluorophenyl-(2,6-difluoro-4-(trans-4-(trans-4-methylcyclohexyl)cy clohexyl)benzoate; PA0 3,4,5-trifluorophenyl-(2,6-difluoro-4-(trans-4-(trans-4-ethylcyclohexyl)cyc lohexyl))benzoate; PA0 3,4,5-trifluorophenyl-(2,6-difluoro-4-(trans-4-(trans-4-(trans-4-propylcycl ohexyl)cyclohexyl))benzoate; PA0 3,4,5-trifluorophenyl-(2,6-difluoro-4-(trans-4-(trans-4-(trans-4-butylcyclo hexyl)cyclohexyl))benzoate; PA0 3,4,5-trifluorophenyl-(2,6-difluoro-4-(trans-4-(trans-4-(trans-4-pentylcycl ohexyl)cyclohexyl))benzoate; PA0 3,4,5-trifluorophenyl-(2,6-difluoro-4-(trans-4-(trans-4-hexylcyclohexyl)cyc lohexyl)benzoate; PA0 3,4,5-trifluorophenyl-(2,6-difluoro-4-(trans-4-(trans-4-heptylcyclohexyl)cy clohexyl)benzoate; PA0 3,4,5-trifluorophenyl-(2,6-difluoro-4-(trans-4-(trans-4-octylcyclohexyl)cyc lohexyl))benzoate; PA0 3,4,5-trifluorophenyl-(2,6-difluoro-4-(trans-4-(trans-4-nonylcyclohexyl)cyc lohexyl))benzoate; PA0 3,4,5-trifluorophenyl-(2,6-difluoro-4-(trans-4-(trans-4-decylcyclohexyl)cyc lohexyl)benzoate.
As an operation system for AM-LCD, a TN display system is employed in which the alignment of a liquid crystalline molecule in the upper and lower substrates is twisted 90.degree.. According to the report by C. H. Gooch and H. A. Tarry in Appl. Phys., Vol. 8, pp. 1575-1584 (1975), the TN display system requires to set a product .DELTA.n.multidot.d of an optical anisotropy (.DELTA.n) and a cell thickness (d).mu.m at a predetermined value (e.g., .DELTA.n.multidot.d=0.5 .mu.m) in order to prevent coloration due to the interference of liquid crystal cell when no voltage is applied, thereby obtaining an optimum contrast. The .DELTA.n of the liquid crystal composition for TFT which is now put to practical use under such limitation ranges from about 0.07 to about 0.11, principally 0.08 to 0.10, for a cell operating near at the first minimum point of the transmission described in the above reference.
In recent years, the application of LCD has been extensive with the development of a notebook type personal computer which is characterized by small size, light weight and portability. LCD for the purpose of portability undergoes the restriction of characteristics by a driving power sourse. Since the long-term use requires a lower power consumption, a liquid crystal composition having a low threshold voltage is demanded. Further, liquid crystals having a low threshold voltage are required to achieve lighther weight and lower cost of the driving power sourse.
With the portability, the development has been considered for the purpose of outdoor use. To make liquid crystals tolerable to outdoor use, it is required that they exhibit a nematic phase in the ranges exceeding the temperature range under use environment. In such circumstances, the liquid crystal compositions for TFT which are now put to practical use have a nematic phase transition temperature wherein the upper limit (clearing point T.sub.NI) is not lower than 60.degree. C. and the lower limit (T.sub.L) is not higher than -20.degree. C.
In such background, Japanese Patent Kokai 2-233626 discloses trifluoro compounds having a relatively high dielectric anisotropy (.DELTA..epsilon.), in which Example 2 illustrates a composition comprising 15% by weight of a trifluoro compound and 85% by weight of a difluoro compound, but the composition has the problems of high threshold voltage, poor compatibility at lower temperature and narrow nematic phase range.
WO 94/03558 discloses a composition comprising trifluoro and difluoro compounds. The compositions illustrated in Examples 1 and 2 are low in the threshold voltage, but have the problems that the clearing point is not higher than 50.degree. C. and .DELTA.n is not higher than 0.06.
Thus, the liquid crystal compositions have been investigated according to various purposes, and under the present circumstances, new improvement has been required.